The present invention relates to a public telephone system constituted by a public telephone set for performing speech communication using a prepaid card, a center apparatus for managing the prepaid card used in the public telephone set, and the like.
In a card public telephone set, speech communication can be performed using a prepaid card (to be referred to as a card hereinafter) storing prepaid value information. When a card is inserted into a telephone set of this type, a call rate as valid information is read from the inserted card, and at the same time, a speech communication time according to the readout call rate is allowed. More specifically, during speech communication after insertion of the card, the call rate is decreased one by one in accordance with an autonomous charge output, i.e., a periodic output from a charge timer incorporated in the public telephone set, which sets a charge time on the basis of an input dial number, or a charge signal detection output which is periodically transmitted to a speech communication line. Upon completion of speech communication, a remaining call rate is recorded in the inserted card, and the card is ejected from the public telephone set.
To prevent illegal use of a card in such a public telephone set, card information including the serial number and the currently used call rate of a card is transmitted to a center apparatus called a card validation system upon completion of speech communication. Upon reception of the card information, the center apparatus retrieves the card and its balance corresponding to the serial number registered in a database in advance. A currently used call rate is subtracted from the card balance, and the result is registered in the database. If the balance obtained by subtracting the currently used call rate from the card balance becomes negative, the card is determined as an illegally used card and registered in a black list, and subsequent use is inhibited.
As described above, the center apparatus for managing cards used in public telephone sets cannot cancel registration of a card which is issued and registered in the database until the call rate becomes zero. In addition, the number of issued cards increases every year, so registration becomes impossible in the future regardless of an increase in memory capacity of the database. For this reason, an appropriate memory capacity cannot be determined. Furthermore, along with an increase in number of registered cards, much time is required for retrieval of a corresponding card.